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We propose nonlinear Faraday rotation as a mechanism for achieving stable polarization mode locking of a soliton laser. We analyze by perturbation theory and beam-propagation simulations the interplay between bandwidth-limited gain, gain dichroism, and linear and nonlinear Faraday rotation.

The subject of this presentation is to show how molecular cluster calculations can lead to a deeper understanding of laser induced processes at surfaces of ionic crystals. Molecular cluster calculations were performed for bulk BaF2 and for different clusters modelling the (111) surface of BaF2. The effect of the surrounding crystal ions have been included by an embedding crystal potential. Calculations for a stoichiometric surface give an electronic structure similar to the bulk, while calculations for a local non-stoichiometric surface show the existence of occupied surface states in the upper half of the bandgap, followed by unoccupied states extending above the ionization limit. These calculations give a qualitative understanding of the experimentally observed resonantly enhanced multiphoton processes of electron and ion emission from the (111) surface of BaF2.

The interaction of laser radiation and surfaces of optically transparent ionic materials is an important problem in surface science as well as in applied optics. For the latter, the question of how optical damage occurs, is of fundamental interest. Since the bandgap of ionic crystals is of the order of 10 eV, for visible light only multiphoton absorption can result in energy deposition into the surface. Structural defects, impurities, and adsorbates can generate surface states which are located somewhere in the bandgap, and can resonantly enhance multiphoton photoemission. This, in turn, can lead to desorption of ions and neutrals at comparatively low intensities. In this contribution we present results of photoemission and desorption studies carried out in ultra-high vacuum with cleaved (111) surfaces of BaF2, at laser intensities in the MW/cm2 range.

We have investigated the wavelength, polarization, and intensity dependence of the electron and ion emission from the (111) surface of BaF2 under irradiation with pulsed tunable laser light in the green and blue spectral ranges. We also carried out cluster calculations to obtain a prediction for the electronic surface structure of BaF2 (111). Based on these results, a qualitative model for the desorption of positive ions from this surface is proposed.

Laser-induced emission of electrons and positive ions as well as optical second harmonic generation (SHG) at the (111) surface of BaF2 yield insight in both the electronic surface structure and the fundamentals of laser damage in optical materials.

Measurements of laser‐induced desorption under ultrahigh vacuum conditions and second harmonic generation on surfaces in air provide evidence for surface electronic states in the band gap of ionic materials. When irradiating BaF2(111) with green laser light narrow resonances of electron and ion emission yields indicate resonance enhancement of multiphoton absorption. Using fundamental light of 532 nm, second harmonic generation behaves different for BaF2(111) and NaCl(111) on one side and CaF2(111) on the other. In the first two cases, its azimuthal dependence proves that the second harmonic yield originates from local dipole interaction at the surface. In contrast, the CaF2(111) surface appears to be more inert and the second harmonic signal originates predominantly from quadrupole interaction in the bulk.